The present invention relates to a fiber treated to enhance permeability of an absorbent structure prepared from such fibers. More particularly, the invention relates to fibers treated with polyvalent metal ion-containing compounds for use in absorbent structures made with such fibers, and absorbent articles containing such absorbent structures.
Absorbent structures are important in a wide range of disposable absorbent articles including infant diapers, adult incontinence products, sanitary napkins and other feminine hygiene products and the like. These and other absorbent articles are generally provided with an absorbent core to receive and retain body liquids. The absorbent core is usually sandwiched between a liquid pervious topsheet, whose function is to allow the passage of fluid to the core and a liquid impervious backsheet whose function is to contain the fluid and to prevent it from passing through the absorbent article to the garment of the wearer of the absorbent article.
An absorbent core for diapers, adult incontinence pads and feminine hygiene articles frequently includes fibrous batts or webs constructed of defiberized, loose, fluffed, hydrophilic, cellulosic fibers. Such fibrous batts form a matrix capable of absorbing and retaining some liquid. However, their ability to do so is limited. Thus, superabsorbent polymer (xe2x80x9cSAPxe2x80x9d) particles, granules, flakes or fibers (collectively xe2x80x9cparticlesxe2x80x9d), capable of absorbing many times their weight of liquid, are often included in the absorbent core to increase the absorbent capacity of the core, without having to substantially increase the bulkiness of the core. In an absorbent core containing matrix fibers and SAP particles, the fibers physically separate the SAP particles, provide structural integrity for the absorbent core, and provide avenues for the passage of fluid through the core.
Absorbent cores containing SAP particles have been successful, and in recent years, market demand has increased for thinner, more absorbent and more comfortable absorbent articles. Such an article may be obtained by increasing the proportion of SAP particles to the cellulose or other matrix fibers in the absorbent core.
However, there are practical limits to increasing the proportion of SAP particles in the absorbent core. If the concentration of SAP particles in an absorbent core is too high, gel blocking can result. When SAP particles distributed through an absorbent core of matrix fibers are exposed to liquid they swell as they absorb the liquid, forming a gel. As adjacent SAP particles swell, they form a barrier to free liquid not immediately absorbed by the SAP particles. As a result, access by the liquid to unexposed SAP particles may be blocked by the swollen (gelled) SAP particles. When gel blocking occurs, liquid pooling, as opposed to absorption, takes place in the core. As a result, large portions of the core remain unused, and failure (leaking) of the absorbent core can occur. Gel blocking caused by high concentrations of SAP particles results in reduced core permeability, or fluid flow, under pressures encountered during use of the absorbent product.
One way to minimize gel block (and maintain core permeability) is to limit the proportion of SAP particles to matrix fibers in the absorbent core. In this way, there is sufficient separation between particles, such that even after the particles have been swollen by exposure to liquid they do not contact adjacent particles and free liquid can migrate to unexposed SAP particles. Unfortunately, limiting the concentration of SAP particles in the absorbent core also limits the extent to which the core can be made thinner and more comfortable. To avoid gel block, commercial absorbent cores are presently limited to SAP particle concentrations of 20% to 50% by weight of the core.
It would be highly desirable to provide an absorbent core capable of bearing a SAP particle concentration exceeding 50% by weight, preferably 50% to 80% by weight, while maintaining core permeability and avoiding the problem of gel block. It would also be desirable to provide an absorbent core, which exhibits improved permeability for a given SAP concentration. At the same time, it is important to be able to blend the matrix fiber and SAP particles into an absorbent core using conventional material shipping and handling processes to provide attractive economics for the manufacture of infant diapers, feminine hygiene pads, adult incontinence pads, and the like.
Other methods for increasing SAP particle concentrations while minimizing gel block, have been directed to modifying the superabsorbent polymer itself. Modification of the superabsorbent polymer usually involves reducing the gel volume of the superabsorbent polymer particles by increasing the crosslinking of the polymer. A crosslinked SAP particle is restricted in its ability to swell, and therefore has a reduced capacity, or gel volume. Although modified SAP particles are less susceptible to gel block, they also absorb less liquid by weight due to their reduced gel volume. Modified SAP particles also tend to be brittle and fracture and crack during or after processing into the final absorbent product. A variety of crosslinkers are known in the art. It is also known to use polyvalent metal ions, including aluminum, during the manufacture of SAPs, to serve as an ionic crosslinking agent. See for example, U.S. Pat. No. 5,736,595.
Crosslinking of SAP particles affects the permeability of the particle, i.e., the ability of liquid to permeate the particle to the center, thereby fully utilizing the capacity of the SAP particle. As used in this specification, SAP particle permeability is distinguished from the permeability of the xe2x80x9ccorexe2x80x9d or absorbent structure. Core permeability refers to the ability of liquid to permeate through an absorbent structure containing SAP particles. As used herein, such permeability is measured by methods including xe2x80x9cverticalxe2x80x9d permeability and xe2x80x9cinclinedxe2x80x9d permeability. A core xe2x80x9cpermeability factorxe2x80x9d may be determined from both vertical and inclined permeability measurements.
A method for improved utilization of the superabsorber is disclosed in U.S. Pat. No. 5,147,343, where particle size distribution of the granules is controlled. By controlling the particle size of the superabsorber and hence the surface area, the rate of fluid uptake can be optimized to the core design. However, the utilization of the absorbent core is reduced at higher concentrations of SAP particles due to gel blocking.
The present invention is directed to absorbent structures including fibers bound with a polyvalent cation-containing compound and superabsorbent polymer particles. The fibers exhibit an ion extraction factor of at least 5%. The present invention is also directed to multi-strata absorbent structures, such as disposable absorbent articles, including the treated fibers and SAP particles.
The present invention is also directed to methods for preparing absorbent structures including the treated fibers; structures including fibers combined with a polyvalent cation-containing compound; and methods for treating or coating SAP particles with polyvalent cation-containing compounds.